In connection with the recovery of hydrocarbons from the earth, wellbores are generally drilled using a variety of different methods and equipment. According to one common method, a roller cone bit or fixed cutter bit is rotated against the subsurface formation to form the wellbore. The drill bit is rotated in the wellbore through the rotations of a drill string attached to the drill bit and/or by the rotary force imparted to the drill bit by a subsurface drilling motor powered by the flow of drilling fluid down the drill string and through downhole motor. In some situations it is desirable to evaluate the formations being drilled while drilling is conducted. Various instruments are run in the drill string to measure parameters that may be used to evaluate the formation(s). Such systems are often referred to as logging while drilling (LWD) and measurement while drilling (MWD).
Acoustic togging tools can be used in MWD and LWD systems to measure acoustic properties of the formations from which images, mechanical properties or other characteristics of the formations can be derived. Acoustic energy is generated by a logging tool and acoustic waves comprising periodic vibrational disturbances resulting from the acoustic energy propagating through the formation or the acoustic togging system are received by an acoustic receiver in the acoustic logging tool, Acoustic waves can be characterized in terms of their frequency, amplitude and speed of propagation. Acoustic properties of interest for formations can include compressional wave speed, shear wave speed, surface waves speed (e.g. Stoneley waves) and other properties. Acoustic images can be used to depict wellbore wall conditions and other geological features away from the wellbore. The acoustic measurements have applications in seismic correlation, petrophysics, rock mechanics and other areas. An effective operation of the acoustic logging tools can be hindered by undesirable noise signals encountered downhole by the logging tools.
Like reference symbols in the various drawings indicate like elements.